Add metrics for close desktop task shortcut

This change adds logging to FrameworkStatsLog for the keyboard
shortcut that closes the focused desktop task. This allows tracking
the usage of this feature.

Bug: 441159915
Test: None
Flag: com.android.window.flags.close_task_keyboard_shortcut
Change-Id: I071dfd3881abec3305c70462b3f13318179625c1